1. Field of the Invention
The present invention relates to an x-ray exposure method directing x-rays generated from an x-ray source to illuminate a resist through a mask.
2. Description of the Background Art
The resolution in x-ray exposure is considered to be determined by two factors that have different causes: resolution limit of optical images determined by Fresnel diffraction, and that of pattern blur (hereinafter simply referred to as blur) caused by exposure of a resist by secondary electrons generated within the resist by the reflection of exposed light. These secondary electrons include photoelectrons and Auger electrons.
The resolution limit L determined by Fresnel diffraction is provided by:L=k×(λ×D)1/2where k is a constant, λ is an exposure wavelength, and D is the distance between an x-ray mask and a wafer. This equation shows that, the shorter the exposure wavelength and the smaller the distance between the x-ray mask and the wafer, the higher the resolution obtained.
On the other hand, blur caused by secondary electrons generated in a resist by x-ray irradiation has been considered to decrease as the exposure wavelength increases, as described by Takigawa in “The Innovation of ULSI Lithography”, published by Science Forum Co. Ltd., 1st edition, at page 222. Thus, it has been considered that the longer the exposure wavelength, the higher the resolution obtained.
Meanwhile, Koji Kise et al in “Suppression of secondary electron blur by using Br-containing resists in x-ray lithography”, Journal of Vacuum Science & Technology B, The Society Through the American Institute of Physics, Vol. 20, No. 6, Nov/December 2002, at pages 2953-2957, describe that blur caused by secondary electrons is suppressed by including an element Br in a resist.
Based on the conventional findings described above, as far as the optimal exposure wavelength is concerned, the influence of Fresnel diffraction on the resolution conflicts with that of blur caused by secondary electrons: exposure wavelength is preferably short in order to decrease the influence of Fresnel diffraction, whereas the exposure wavelength is preferably long in order to decrease the influence of blur caused by secondary electrons.
On account of these influences, for example when the distance between an x-ray mask and a wafer is 5 μm, it has been considered that around 8 angstroms is the optimal exposure wavelength and the resolution deteriorates when the exposure wavelength is shorter than this, as shown in the graph of FIG. 3 in “The Innovation of ULSI Lithography”, at page 222.